Whereas thermoplastics have had widespread use in reciprocating-screw injection-molding machines and processes for about three decades, the largest volume of thermoplastic material is consumed by low-precision moldings from the so-called commodity thermoplastics families (i.e., polyolefin, polystyrene, vinyl). More recently, and in the last decade especially, higher-degree-of-difficulty applications of new, higher-performing engineering thermoplastics have evolved These engineering thermoplastics' applications have extended the range of plastic molding into what has traditionally been the domain of nonplastic materials (such as glass, in the case of optical materials; or precision-machined metals, ceramics or reinforced thermoset composites, in the case of electronics uses). Compared to the more common high-volume thermoplastics polymer families, these new engineered thermoplastics typically are characterized as being:
(1) Substantially more expensive per pound ($2.00-$20.00 per pound);
(2) Possessed of a relatively higher melt viscosity at their recommended processing range;
(3) Usually amorphous polymers (thus, lacking the easy fluidity of crystalline polymers above their melt temperatures);
(4) Molded at temperatures commonly 100-200 degrees higher than the commodity thermoplastics.
These high-performance, difficult-processing thermoplastics are required to meet stringent molded-product quality standards, which in turn depend greatly upon the melt's quality, as delivered into the mold cavity
These optimal melt quality requirements are detailed in Applicants' co-pending patent application, Ser. No. 06/929,399, filed Nov. 7, 1986 now abandoned, regarding improved reciprocating-screw plastication. Even achieving optimal melt quality as it leaves the machine's nozzle, however, is not sufficient to assure its suitability when it actually enters the mold cavity, and this is especially true for the more complex melt delivery systems inherent in multicavity molds. (Perhaps, as appears to be the case in plastication processes, less attention has heretofore been paid to melt quality in injection molding than in extrusion processes.) Too often the mold operator increases the temperature as a simplistic answer to such problems, often to the point where the increased heat history given the plastic can no longer be tolerated.